This application claims the benefit of Korean Patent Application No. 2000-79354, filed on Dec. 20, 2000, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a color liquid crystal display device without a color filter.
2. Description of the Related Art
As our information-oriented society rapidly develops, display devices are increasingly developed. The display device processes and displays a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display device field. However, a flat panel display devices having small size, lightweight, and low power consumption are actively being researched in order to meet the needs of the times. Accordingly, a thin film transistor-liquid crystal display (TFT-LCD) device that has high color quality and small size is developed.
The LCD device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. The orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling the applied electric field a desired image can be produced.
FIG. 1 is a schematic perspective view of a conventional TFT-LCD device.
The TFT-LCD device includes upper and lower substrates 5 and 22, and a liquid crystal layer 14 interposed therebetween. The upper and lower substrates 5 and 22 are sometimes referred to as a color filter substrate and an array substrate, respectively. On a surface facing the lower substrate 22, the upper substrate 5 includes a black matrix 6 and a color filter layer 7. The color filter layer 7 includes a matrix array of red (R), green (G), and blue (B) sub-color filters that are formed such that each sub-color filter is bordered by the black matrix 6. The upper substrate 5 also includes a transparent common electrode 18 over the color filter layer 7 and the black matrix 6. On a surface facing the upper substrate 5, the lower substrate 22 includes an array of thin film transistors (TFTs) shown as a xe2x80x9cTxe2x80x9d that act as switching devices. The array of thin film transistors is formed to correspond with the matrix of sub-color filters. A plurality of crossing gate and data lines 13 and 15 are positioned such that a TFT is located near each crossing of the gate and data lines 13 and 15. The lower substrate 22 also includes a plurality of pixel electrodes 17, which are made of a transparent material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO). Each pixel electrode is disposed in a corresponding area defined between the gate and data lines 13 and 15 and often referred to as pixel regions P.
In LCD devices an electro optic effect of the liquid crystal material is a phenomenon where an electro optic modulation occurs by the change of the optical property of the liquid crystal material, that is, an alignment state of the liquid crystal material turns to another alignment state by an applied electric field.
The LCD devices utilize the electro-optic effect of the liquid crystal mateiial and can be divided into 4 display types. The following is description of the LC operating modes using various electro-optical effects, including a twisted nematic (TN) mode, a guest host (GH) mode, electrically controlled birefringence (ECB) effect and ferroelectric liquid crystal (FLC) mode. The display type can be selected and used according to a design characteristic of the liquid crystal panel.
Recently, various methods to express colors without the color filter by using wavelength property of light due to the birefringence of the liquid crystal material have been suggested. Among them are a method using the ECB effect and will be explained with the color TFT-LCD device using the ECB mode. In the ECB mode LCD device, the pretilt angle is nearly 90xc2x0 and the liquid crystal molecules are aligned parallel to the substrate by the applied voltage such that a transmittance of light can be controlled.
FIGS. 2A and 2B are cross-sectional views of a conventional ECB mode reflective LCD device and shows the operation of liquid crystal molecules when an applied voltage is off or on, respectively.
As shown in FIG. 2A, upper and lower substrates 31 and 33 are spaced apart from each other with a specific gap (d) and liquid crystal molecules 35 are interposed therebetween. An upper polarization plate 39 is formed on an outer surface of the upper substrate 31 and as a result of the upper polarization plate only light parallel to the transmittance axis of the polarization plate can be emitted to the exterior of the display. The liquid crystal molecules 35 are aligned with an arbitrary pretilt angle xcex8p between about 0xc2x0 and 90xc2x0, preferably an angle between 60xc2x0 and 85xc2x0 considering a property of viewing angle and gray scale inversion. To acquire the desired pretilt angle, orientation films 37 and 38 are formed on each facing surface of the upper and lower substrates 31 and 33 and a rubbing process of the orientation films 37 and 38 is performed.
As shown in FIG. 2B, when a voltage is applied to the upper and lower substrates 31 and 33, the liquid crystal molecules 35 are aligned with another angle xcex8m. The light is polarized as it passes through the polarization plate 39, the upper substrate 31, the liquid crystal molecules 35 and a reflection plate 41. The transmittance of the light changes according to the angle between the light axis of the liquid crystal molecules 35 and the transmittance axis of the polarization plate 39 when the voltage is applied to the LCD device.
The transmittance of the ECB mode LCD device is dependent on the wavelength in contrast with the transmittance of the TN mode LCD device.
An equation for the transmittance (T) of the ECB mode reflective LCD devices is as follows. The angle between the light axis of the liquid crystal molecules and the transmission axis of the polarization plate is assumed to be 45xc2x0 and the LCD device includes the polarization plate, the liquid crystal layer and the reflection plate from the top.
T=cos2(2xcfx80dxc2x7xcex94n(xcex)/xcex)/2xe2x80x83xe2x80x83(Equation 1)
where d is a cell gap between the upper and lower substrates 31 and 33, xcex94n is a refractive index anisotropy, xcex is a wavelength and dxc2x7xcex94n is a retardation. As shown from the equation 1 the transmittance is a function of retardation (dxc2x7xcex94n) for a specific wavelength (xcex) of the incident light.
FIG. 3 shows the transmittances of red (R), green (G) and blue (B) light 43, 45 and 47, respectively, for the conventional ECB mode reflective LCD device as a function of the effective retardation (dxc2x7xcex94neff), which can be calculated by using the equation (1). The colors of transmitted light continuously change in a series of white, black, blue, green and pink according to the effective retardation (dxc2x7xcex94neff) by the applied voltage.
However, in the conventional ECB mode LCD device, since the viewing angle is too narrow, the color is different according to the viewing direction. Moreover, since the transmittance curves of the colors have the peaks of the same height, the number of the expressible colors is limited.
Accordingly, the present invention is directed to a color liquid crystal display device and manufacturing method thereof that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a color liquid crystal display device and a manufacturing method thereof that can realize multiple colors or full color without the use of a color filter.
Another advantage of the present invention is to provide a color liquid crystal display device that has a wide viewing angle.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. These advantages and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a color liquid crystal display device includes upper and lower substrates facing and spaced apart from each other, a liquid crystal layer interposed between the upper and lower substrates and initially aligned parallel to the substrates and upper and lower polarizers respectively disposed on an outer surfaces of the upper and lower substrates, wherein the liquid crystal layer is re-aligned by an applied voltage and a transmittance of the liquid crystal layer for a specific wavelength is changed according to a change of an angle between an light axis of the liquid crystal layer and a transmission axis of the polarizers so that the device can display multiple colors.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.